Austenitic stainless steels have superior corrosion resistance and heat resistance so that they are used in various fields such as chemical plants, petroleum plants, nuclear plants, aircraft, vessels, automobiles, buildings, electric appliances, home appliances and so on. Since, besides that, austenitic stainless steels are non-magnetic, they are also used in such parts where both strength and non-magnetism are required, e.g., mechanical parts of various electronic meters, a micro-shaft of a video tape recorder (VTR), a magnet roller of a printing machine, a shaft and a case of a magnetic valve.
The 18-8 stainless steel (JIS [Japanese Industrial Standards] SUS 304) and high-manganese stainless steel (ASTM 205) are typical non-magnetic austenitic stainless steels. The 18-8 stainless steel is a low carbon steel including about 18% chromium and about 8% nickel. Since the chemical composition is close to the border of the austenite region in the phase diagram, the 18-8 stainless steel is in a semi-stable state. Therefore, martensite is easily generated in a cold forming process and .delta.-ferrite is easily generated when the balance in the chemical composition is fluctuated. Since martensite and .delta.-ferrite are ferromagnetic, austenitic stainless steel including these phases acquires slight ferromagnetism, which loses one of its important merits.
As naturally understood, permeability of the austenitic stainless steel is related to the amount of these ferromagnetic phases. In order to assure the non-magnetic quality of their products, stainless steel manufacturers and part makers using non-magnetic stainless steel measure the permeability of the product. When the measured permeability value is greater than a predetermined value, the microstructure of the material steel is examined to determine the cause of the high permeability. If it is mainly caused by .delta.-ferrite, the composition of the stainless steel is modified by adding austenite forming elements (i.e., elements that stabilize the austenite phase) such as nickel (Ni), carbon (C), nitrogen (N), etc., or the steel is heated for a long time at a high temperature before rolling to decompose the .delta.-ferrite. If the main cause of the high permeability is determined to be martensite, the austenite forming elements as described above are added or the steel making process is properly modified.
In order to examine the microstructure of metal material, one should prepare a sample for microscopic inspection: first cut out a small piece of metal sample, polish it, and etch it with a proper acid solution. The determination of the martensitic phase and .delta.-ferrite phase is difficult and the quantification of the content in the microscopic field requires complex calculations. That is the reason why convenient and short time measurement of permeability (which is related to the amount of the ferromagnetic phase) of metals has long been desired.
There are some other methods for measuring permeability of metal material. One is a magnetic balancing method in which an object sample piece and a reference non-magnetic sample piece are balanced and the same magnetic field is applied to both sample pieces. The magnetic balancing method is difficult to use in the field and the measurement cannot be made in a short time.
The inventors of the present invention have already developed a permeability measuring instrument (permeameter) that can be used in the field and can measure the permeability non-destructively (i.e., without preparing a small inspection sample) and in a short time (Japanese Published Examined Patent Application No. 20114/1984).
Recent trend in miniaturization of electronic devices requires smaller mechanical parts. For example, a micro-shaft used in a VTR is becoming smaller as the VTR itself is shrinking. The demand for non-magnetism is also becoming strict: the permeability value is required below 1.01 or 1.02. That is, a new permeameter is required that can measure a small object with high sensitivity (e.g., resolution of smaller than 0.001) non-destructively and easily.
The working principle of the permeameter described above is the magnetic induction phenomenon of an object metallic material. The probe of the permeameter is made from a ferromagnetic core rod and an exciting coil and two detecting coils wound thereon.
For example, for a cylindrical sample, the diameter needs more than 10 mm to correctly measure the permeability value with the permeameter described above. Microshafts used in current small-size VTRs are about 2 mm in diameter, and the demand for the non-magnetism is, as described above, below 1.01 or 1.02 in the permeability value.
In another prior art, an exciting coil and a detecting coil are combined to make a unit, and two such units are wound on the core rod to make a differential-transformer-type probe. In one type of the permeameter, in order to align the magnetic flux in the core rod straight as long as possible, the two coil units are placed at both ends of the core rod so that the inter-coil distance be as large as possible. In this type of permeameter, it is especially difficult, as described later, to measure the permeability value of a small sample accurately.